The invention relates to a sputtering system for cathode sputtering apparatus, consisting of a cathode base with a target of the material to be sputtered, of a dark-space shield separated by an air gap from the cathode base body, and of an anode disposed in the area of the margin of the target and having at least one open groove whose opening is disposed in a side of the anode that is not in line of sight with the target.
In cathode sputtering apparatus, the anode, as its name signifies, serves for closing the circuit with respect to the cathode. The anode serves for capturing or for the removal of the electrons pertaining to the sputtering process and involved in this typical glow discharge.
In many known sputtering systems, the vacuum chamber that is at ground potential--the so-called receptacle--serves as the anode. However, for reasons of the control of the process, it has proven desirable to provide a separate anode directly adjacent the cathode or target, which can be connected directly to the ground or can be placed at the ground potential, or which is set at a potential level--usually positive--with respect to ground. In the present case, too, what is involved is a sputtering apparatus having an additional anode present.
Now, by no means does the material sputtered from the cathode travel only in a straight line and only in a direction perpendicular to the sputtered surface of the target, to the substrate which is movable or fixed with respect to this sputtered surface. Instead, the coating material is distributed through the space, so that it precipitates or condenses more or less strongly also on the anode.
Insofar as the coating material is electrically conductive, which is regularly the case in the sputtering of metals in a neutral or nonreactive atmosphere, the transition resistances in the circuit between cathode and anode are not negatively affected even in the course of lengthy coating processes. In numerous applications in electronics and optics, and in the coating of large-area window glass, however, coatings of nonconductive materials--so-called dielectrics--are required. These coatings can be produced both by the sputtering of oxide-forming metals in a reactive atmosphere, as well as by sputtering targets made from the oxides themselves. Experience has shown that inevitably a more or less large proportion of the sputtered material condenses on the anode in the form of the reaction product, and there forms an insulating surface coating which, as its thickness increases or as time goes on, increasingly reduces the anode current. This results not only in uncontrollably unstable operating conditions, but also in extremely undesirable flashovers when highly localized surface charges form on the insulating surface coating of the anode, which from time to time discharge and gretly interfere with the sputtering process.
There has been no lack of attempts, therefore, to remove the anode as far as possible away from the glow region in the vicinity of the target surface. For example, German Federal publication OS No. 24 17 228 recommends mounting the anode outside of the glow discharge as well as outside of the direction of the migration of the sputtered particles of the target material. Aside from the fact that this goal is virtually unattainable, this impairs or delays the entry of the electrons into the anode, so that the anode draws to itself only a small part of the cathode current. The greater part of the cathode current then necessarily flows to the inside surfaces of the vacuum chamber that is at ground potential, or to fittings in the vacuum chamber that are at ground potential, including especially the substrate holder which, in the case of support material in continuous web form can also be a rotating cylinder. The consequences are, again, unstable operating conditions which are unacceptable in a continuous process.
The anode, therefore, has been taken out of its sheltered place and brought to the immediate vicinity of the target, in the hope that, by a high temperature level that will thus necessarily be produced (the glow discharge attacks the anode directly), either the coating of the anode will be prevented, or the anode will be "cleaned." It is also virtually impossible, in this case, to sustain stable operating conditions over a long period of time in the sputtering of insulating or dielectric materials. This is because the requirements of long anode life or long-term constant operating conditions on the one hand, and very good electrical coupling of the anode on the other are virtually opposed to one another.
Through German Federal publication OS No. 22 43 708, it is known to dispose an annular anode in the marginal area of flange of an otherwise substantially cylindrical cathode. In one of the annular anode surfaces, which is in line-of-sight relationship to the target material, concentric grooves are provided, which serve to improve the adherence of the sputtered material impinging upon the anode. This does make it possible to a gret extent to prevent the material, which is undesired at this point, from peeling off and falling after it reaches a certain thickness; it is impossible, however, to forestall increasing insulation by the impingement of insulating materials (dielectrics or even appreciably to suppress it. It is precisely interlocking with the anode surface that assures the growth of an especially thick insulating layer, which necessitates frequent cleaning of the anode.
German Federal OS No. 26 36 293 discloses a cathode sputtering apparatus whereby different materials are to be sputtered either simultaneously or successively. To prevent a so-called cross-contamination of the target material by the condensation of target material of a different cathode, shields are disposed between the cathodes or electrodes on the one hand and the substrates or substrate holder on the other, these shields being at ground potential and consequently exercising an anode function to a certain extent. This anode function, however, deteriorates in the course of time, whenever the sputtering of dielectrics is involved. A labyrinth formed by grooves in the faces of the shields cannot replace or sustain the anode function, because the grooves are too far away from the actual sputtering surface and from the area in which the glow discharge is burning which produces the sputtering.
The invention is therefore addressed to the problem of devising a sputtering system of the kind described in the beginning, which will assure stable operating conditions over long periods in the coating of substrates with insulating materials, and in which the anode will draw to itself a high proportion of the cathode current.